How to: set the "world" environment colour to white
Click the "Use nodes" just above the color input. It will replace that simple color input to some nodes, giving you access to the color and strength of the environment. Make sure the color is pure white, and the strength at least at 1.

If not enough, then it might be some other issue(s). The two I can think of right now:

Ray visibility: in the World's settings there is a Ray Visibility panel, make sure everything's checked:



Color Management: Go to the Scene tab, Color management panel, make sure you have nothing fancy: 



If it doesn't solve your issue, then a deeper digging may be required, and you should provide your .blend file.